i think it's just because people restore it from their old iPhone backups...
when i went from my 2G to 3GS i also had random problems when i restored from the old backup but a fresh restore with setting it up as a new phone fixed everything :P

But you lose all your app data and SMS database doing it that way dont you?

I have hung up on people on several occasions because of the faulty proximity sensor. It appears, they put the sensor right above the ear piece in this phone unlike (correct me if I am wrong) the iPhones where it was adjacent to the ear piece. Now you really have to press the phone's ear piece to your face for the screen to turn off.

Its an iOS4 with restore from backup problem. I had it on my 3Gs using iOS4 GM firmware... Start as a fresh iPhone then sync as normal youll be fine. :-) Took me a couple restores to figure out tho!! ha ha

I'm on a 3GS and as soon as I installed 4.0 with jb, my screen would go blank every time i held it to my face and it would not come back easily. I read here to reset all my setting and it WORKED! I tried resetting network settings and nothing, reset all settings worked great!

anyone else having this problem go to settings>general>reset> reset all settings. Worked for me

I'm on a 3GS and as soon as I installed 4.0 with jb, my screen would go blank every time i held it to my face and it would not come back easily. I read here to reset all my setting and it WORKED! I tried resetting network settings and nothing, reset all settings worked great!

anyone else having this problem go to settings>general>reset> reset all settings. Worked for me

i tried with limited success. It doesn't seem to ba as sensitive as before but the issue persists.

Well, today I just got my 32gb i4. Was in and out of the store in 7 minutes. I'm fortunate enough to live near the largest Apple store in the U.S. So far no reception issues while holding it, and no yellow spots. Hope this proximity sensor issue stays away as well. This screen is frickin' amazing

Its an iOS4 with restore from backup problem. I had it on my 3Gs using iOS4 GM firmware... Start as a fresh iPhone then sync as normal youll be fine. :-) Took me a couple restores to figure out tho!! ha ha

Like i said in another post, dont you lose app data and SMSs by restoring as new and then syncing?

To me this is good... I've had my phone off a full charge for 8 1/2 hours now. I've talked on the phone for about 2 1/2 hours and "played" around on the Internet for maybe 3 1/2 hours total during that time. Brightness at 50%. I'm at 12% battery life left. Seems good to me compared to my 3GS.

But I guess none of that matters since apple is sending me another phone. Even after a fresh restore my screen would turn on and off while talking.

Wow and the Iphone 4 problems just keep on coming. Those of u that are holding off, u might wanna keep holding off until next June. Something tells me there will be alot of things fixed in the next release. And well of course as always, alot of things screwed up in the next release haha

3GS 32GB unlocked, jailbroken on iOS4 and mine works perfect. So for sure its not the iOS4 causing it. As for the ole ladies Iphone 4, she put it on ebay and decided to use her other phone. Thats the first time i ever heard her cus over an Apple product. And its even funnier because i told her from the beginning that it wasnt worth getting until later.